Insulating ring for particle accelerator tubes and acceleration tube including the same

ABSTRACT

An acceleration tube having insulating rings shaped on the inside surface to relieve stress at the line defined by the intersection of the insulator, electrode and evacuated tube interiors and to maintain insulating properties during and after tube sparks. Annular cuts in the insulators at the defined lines have a width to height ratio of approximately 2.5/1.0, the horizontal width falling within the range of 0.225 inch to 0.275 inch, and the vertical height falling within the range of 0.75 inch to 0.125 inch. One or more annular grooves or a spiral groove may be provided between the cuts, with a depth greater than the cut width.

United States Patent 1 Charpentier [451 Oct. 9, 1973 [76] Inventor: Robert R. Charpentier, 21 Miland Ave., Chelmsford, Mass.

[22] Filed: Aug. 19, 1971 [21] Appl. No.: 173,215

[52] U.S. CI 313/63, 174/140 R, 174/212 [51] Int. Cl. H05h 7/00 [58] Field of Search 174/140 R, 141 R,

174/212; 310/5, 6, 7; 313/63, 243, 244, 246, 249, 250, 268, 82 R, 82 BF; 328/233 [56] References Cited UNITED STATES PATENTS 1,760,983 6/1930 Gordon et a1 174/212 X 2,182,185 12/1939 Trump 174/141 R X 2,376,439 5/1945 Machlett et a1 313/246 2,460,201 1/1949 Trump et al. 174/140 R X 2,521,426 9/1950 Trump et al. 174/140 R X 3,349,283 10/1967 Krefft ..3l3/250X OTHER PUBLICATIONS Alien Property Custodian Application of Thommen, Ser. No. 335,154, published June 22, 1943.

Primary Examiner-Laramie E. Askin Attorney-Russell & Nields [57] ABSTRACT An acceleration tube having insulating rings shaped on the inside surface to relieve stress at the line defined by the intersection of the insulator, electrode and evacuated tube interiors and to maintain insulating properties during and after tube sparks. Annular cuts in the insulators at the defined lines have a width to height ratio of approximately 2.5/1.0, the horizontal width falling within the range of 0.225 inch to 0.275 inch, and the vertical height falling within the range of 0.75 inch to 0.125 inch. One or more annular grooves or a spiral groove may be provided between the cuts, with a depth greater than the cut width.

2 Claims, 7 Drawing Figures PAltNIED 91975 3.764.838

4 FIG. 2

ELECTRIC FIELD STRENGTH BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to charged particle accelerators, and in particular to acceleration tubes employed therein for sustaining a high voltage produced by an electrostatic generator.

2. Description of the Prior Art In particle acceleration tubes metal electrode disks are sandwiched between glass insulating rings to distribute the applied voltage evenly along the length of the tube. The greater the voltage difference that can be maintained between each pair of adjacent electrodes, the more voltage that can be applied across the whole tube, resulting in increased particle acceleration. A limiting factor on the magnitude of voltage achievable between any two electrodes, and hence on the over-all capability of the accelerator, has been the tendency toward electrode flash-overs across the inside surface of the dielectric rings. A flash-over will frequently result in a cascade of charged particles between opposite electrodes of such severity as to vaporize exposed electrode metal. The vaporized metal then lodges on the dielectric surface, lowering its insulative effectiveness and further reinforcing the propensity to flashover. Eventually all operation of the accelerator must cease while the affected insulating rings are either cleaned or replaced.

SUMMARY OF THE INVENTION In order to alleviate the above problems and improve tube performance and life, the present invention provides a unique inside insulator profile that significantly lessens the chance of surface flashover. In accomplishing this, the electric field in the vicinity of the insulatorelectrode-vacuum interface is constrained under the influence of the insulator profile to a pattern less conducive to flashovers. Specifically, an annular portion is cut away at the two corners of the insulating rings where the adjacent electrodes face the vacuum at the interior of the tube. The width of the cut is 0.25 inch and the height 0.10 inch, each dimension having a tolerance of up to :t 0.025 inch.

A further enhancement in electrical insulative properties is achieved by cutting one or more grooves into the insulator face between the above-described cuts. The grooves should be deeper than the cuts, and may permissibly be either annular and discrete or in a continuous spiral. In addition to further improving the electric field pattern, the inner parts of the grooves are shielded from any vaporized electrode metal, preventing any continuous conductive layer from building up across the insulator surface.

Further features and advantages of the invention will appear in theaccompanying detailed description.

DRAWINGS FIG. 1 is a somewhat diagrammatic view in vertical central section of an electrostatic particle accelerator having an accelerator tube embodying the invention;

FIG. 2 is a segmented vertical sectional view of the accelerator tube of FIG. 1;

FIG. 3 is an enlarged sectional view of the left hand profile of two electrode disks and an insulating ring shaped according to the invention;

FIG. 4 is a graph comparing a typical electric field pattern encountered in the prior art to the pattern with the current invention;

FIGS. 5 7 are enlarged sectional views showing various embodiments of the insulating ring profile.

DETAILED DESCRIPTION OF THE INVENTION Referring first to FIG. 1, in a typical electrostatic accelerator a tank 2 is bolted to a base plate 4 at ground potential and filled with a gas under high pressure to insulate the high voltage portions of the accelerator. Electric charge is conveyed to a high voltage terminal 6 in a conventional manner such as by an endless belt (not shown). The terminal 6 sits upon an acceleration tube 8 the interior of which is evacuated and serves as a causeway for charged particles undergoing acceleration. Voltage stress is distributed evenly by equipotential rings 10 surrounding the tube 8 and coupled directly thereto.

As shown in FIG. 2, the acceleration tube 8 consists of alternating metal electrode disks l2 and usually glass insulating rings 14 joined together in a conventional manner of which nothing further need be said except as relates to the unique features of the present invention. The electrodes 12 and insulators 14 are both open at the inside for the passage of charged particles, the electrodes 12 overlapping the insulating rings 14 and extending further into the tube 8. When the accelerator is energized, each succeeding electrode 12 is raised to a particular voltage level, which level decreases uniformly with distance from the high voltage terminal 6.

Tube sparks during the conditioning period have proven to be a prime souce of flashovers across the surface of the insulating rings 14, with a consequent deterioration in performance eventuating in shutdown and overhaul. Although the sparks occur between electrodes 12, towards the middle of the tube, away from the insulators 14, they rapidly cause the electric field existing in the vacuum between electrodes to drop to zero. The electric field within the insulators l4 falls more slowly, resulting in a considerable electrical stress during the period of lag at the lineal interface of each electrode 12, the adjacent insulator l4 and the tube vacuum. This interface shall henceforth be referred to as the triple line 16. The build-up of stress at the triple line 16 effects a disturbance in the negative charge distribution existing along the insulator 14 surface. Localized charge accumulations and voids are created, leading eventually to a surface fiashover and electrode vaporization described above.

It has been found that by cutting away an annular portion 18 at the inner side of the glass insulating rings 14 adjacent to the adjoining electrodes 12, said cuts conforming to a particular set of dimensions, the electrical stress at the triple line 16 during transient electric field conditions can be significantly reduced. As a result, insulating surface flashovers are inhibited and the problem of electrode vaporization is substantially reduced, permitting a longer insulator life between tube overhauls and replacement.

In FIG. 3 is shown an enlarged view of an insulating ring 14 encompassing the unique features of the invention. The cut away portion is described by a floor 20 and a back wall 22 at substantially right angles to one another which, together with the overhanging electrode 12, form an open ended rectangle opening into the evacuated interior of the acceleration tube. When the floor 20 and back wall 22 are of the dimensions to be specified herein, it has been discovered that during tube sparks there is a gradual transition from the zero electric field state in the vacuum between electrodes 12 to the non-zero electric field state in the insulating ring 14. This contrasts with an abrupt drop in field strength at the insulator-vacuum interface encountered in previous designs and permits tube operation with a much lower stress at the triple line 16.

The electric field pattern during a tube spark in which the reduction in field strength inside the insulator lags that in the vacuum is shown in FIG. 4. Electric field strength is represented along the ordinate. Distance is shown on the abcissa, taken along a radial line of. the acceleration tube, beginning on the left at a point in the insulator well' away from the vacuum and continuing to the right through the triple line 16 to a point in the vacuum between the electrodes 12 that is well away from the insulating ring 14. The direction X along the abcissa indicates a traverse of the radial line from the exterior of the tube towards the middle, with the triple line 16 located at A. In prior designs, as shown by the dashed lines, a sharp discontinuity in field strength existedat the triple line, producing the aforesaid harmful stress. Such prior art designs generally were similar to the structures shown in US. Pat. No. 2,521,426 to Trump et al., and comprised insulating rings of the same general shape as that shown in FIG. of the present application, except that the dimensions corresponding to those identified by the reference numerals 22 and in FIG. 5 of the present application were equal to each other, representative dimensions being one-fourth inch by one-fourth inch and one-eighth inch by one-eighth inch. With the present configuration, however, shown as a solid line, the gradient at the triple line 16 is reduced because the electric field inside the insulating rings 14 begins to taper off before reaching the triple line 16, and the field in the vacuum rises gradually as the triple line 16 is approached. The triple line stress is therefore signific'antly'reduced.

To achieve these results, the ratio of the width of the floor 20 to the height of the back wall 22 must be approximately 2.5/1, with the absolute floor 20 width about 0.25 inch and the back wall 22 height about 0.10 inch. Some leeway is permissible, but for the advantages contemplated by the invention the width of the floor 20 should be within the range of 0.225 inch to 0.275 inch, and the height of the back wall 22 should be within the range of 0.075 inch to 0.125 inch. It has been found that a close adherence to the exact dimensions of 0.25 inch and 0.10 inch gives best results.

The electric field gradient can be further reduced by annular grooves 24 cut into the inner glass face by any convenient means at intervals between the cut-away portions. The grooves 24 are cut deeper into the glass than the floor-20, forcing electric field lines inside the glass away from the vicinity of the triple line 16. For an insulating ring with typical dimensions of one inch in height and five-eighths inch between inside and outside radii, mechanical strength considerations generally limit the depth of the grooves 24 to about 0.35 inch. Although the over-all strength of the insulating rings 14 is of course lessened, the interior grooves 24 create a bellows effect between the insulator and the adjacent electrodes 12 which actually reduces mechanical stress at the triple line 16. The number of grooves 24 is limited only by the static pressure that must be supported by the insulating ring 14. Two grooves (FIG. 3) or more may be employed, or only a single groove 26, as shown in FIG. 6. As the number of grooves increases, the permissible deviation from the floor 20/back wall 22 ratio of 0.25 inch/0.10 inch is lowered.

Another advantage afforded by the use of interior grooves is seen from the tendency of any electrode 12 metal that should happen to become vaporized to fall upon the vertical surface of the insulating ring 14 facing the center of the tube before it can reach the inside of the groove. This shielding effect prevents any continous buildup of conducting material along the insulator surface between electrodes 12. However, although the provision of interior grooves enhances the beneficial effect of the unique cut-away portion 18, it is to be understood that they are not essential for the improved operation described, and that substantial benefits can be achieved by the use of an insulating ring 14 of the proper dimensions with no such grooves, as illustrated in FIG. 5.

Referring now to FIG. 7, a spiral groove 28 is cut into the insulator surface between the floors 20 of the cut away portions. In addition to the advantages of the embodiments above, charge on the insulator 14 is permitted access to uniform leakage by the spiraled, continuous surface. Continuous leakage preserves a uniformity in electric field in the insulator surface region necessary to prevent localized accumulation of charge and consequent fiashover. A pitch of six turns to the inch is preferred, but other degrees of pitch may also be used. The factors involved in determining depth for annular grooves also apply to a spiral, and the groove will generally be about 0.35 inch in depth.

As in prior tubes, the insulating rings 14 are joined to the electrodes 12 by a layer of adhesive, the exposed portion of which is also subject to vaporization during a surface discharge. In order to maintain the reduced field gradient described above, the adhesive surface is concave inwards with respect to the triple line 16.

The various cuts described on the inside of the insulating rings 14 of necessity sacrifice over-all ring strength to achieve the enhanced electrical properties. As the rings 14 are generally supported in cantilever fashion, it has been found necessary to remove all corrugations from their external surfaces in order to increase mechanical strength, thus losing the improved electrical properties on the outside. The superior insulating qualities of the pressurized gas surrounding the tube 8 permit the use of a straight exterior, however, without a corresponding injury to tube performance.

Having now described the basic principles of my invention along with several embodiments, various modifications may suggest themselves to those skilled in the art. For example, the number of grooves may be varied from the illustrations herein, or the grooves might be susceptible of deeper cutting. It is therefore my intention that the described embodiments be taken in an illustrative sense, and that the invention be limited only in terms of the appended claims.

I claim:

1. In a particle accelerator having a source of charged particles and a high voltage terminal, an acceleration tube in electrical communication with said high voltage source comprising a multiplicity of alternating electrode disks and insulating rings, an annular cavity formed at the inner circumferential junctures of adjacent disks and rings, each said cavity founded on a first side by said electrode disk, on a second side opposite to said first side and on a third side between the first and second sides by said insulating ring, and on a fourth side by the open interior of the tube, the cross-sectional length of said second side and said third side falling respectively within the ranges 0.225 inch to 0.275 inch and 0.075 inch to 0.125 inch, each said insulating ring having at least one annular groove per inch of axial 

1. In a particle accelerator having a source of charged particles and a high voltage terminal, an acceleration tube in electrical communication with said high voltage source comprising a multiplicity of alternating electrode disks and insulating rings, an annular cavity formed at the inner circumferential junctures of adjacent disks and rings, each said cavity founded on a first side by said electrode disk, on a second side opposite to said first side and on a third side between the first and second sides by said insulating ring, and on a fourth side by the open interior of the tube, the cross-sectional length of said second side and said third side falling respectively within the ranges 0.225 inch to 0.275 inch and 0.075 inch to 0.125 inch, each said insulating ring having at least one annular groove per inch of axial length extending from the inner circumferential surfaces of said insulating rings between said cavities into the rings, by an amount deeper than the cross-sectional length of said second cavity side.
 2. The acceleration tube of claim 1, wherein the cross-sectional length of the second side of said cavity is substantially equal to 0.25 inch, and the cross-sectional length of the third side is substantially equal to 0.10 inch. 